The scene could have been lifted from an Isaac Asimov novel: a planet a little bigger than Earth orbiting a star halfway to the center of the Milky Way.

But it's for real - among the latest in a string of discoveries by astronomers searching for alien solar systems and clues to how they form. Astronomers studying the puzzling variety of newly discovered planets ultimately hope to learn how many Earth-like planets are out there, some perhaps hosting life.

Wednesday, a team of astronomers announced that it has discovered a planet some 21,500 light years away. Its mass falls between Earth's and Neptune's - the smallest "extrasolar" planet yet found. It orbits its parent star - a tiny speck with only 22 percent of the sun's mass - at roughly twice the distance that Earth orbits the sun.

The discovery is significant because the new solar system is so far away, signifying the greatly expanded volume of space astronomers can search. It is also notable for the team's technique, which is naturally attuned to spotting smaller planets, researchers say.

While the newfound planet is too distant for detailed study, it reveals enough information to become a useful part of the planetary census astronomers are conducting. In the past 12 years, astronomers have bagged at least 170 planets, according to records at the Paris Observatory.

With so many planetary confirmations, "it's kind of tempting at this stage for people to go, 'Ho-hum, they've found another planet, big deal,' " says Stephen Kane of the University of Florida, a member of the team of astronomers from observatories worldwide who made the discovery. "But we're starting to cross new thresholds" that allow researchers to spot smaller planets at greater distances from their host stars.

The astronomers' Holy Grail is to find Earth-like planets that orbit stars at distances that leave them not too hot and not too cold, but just right for liquid water to exist on the surface. Those become the potential sites for life outside our solar system.

Astronomers expect to find a large number of Earth-like planets, says Dr. Kane. But he notes their estimates are extrapolated from the planets discovered so far.

"The fact of the matter is we don't really know," he says. "Until we start getting enough detections of these kinds of planets, it's going to be impossible to say" how many are out there and in the right orbits.

Most planets found so far have been spotted via the wobble they impart to their suns as they orbit. A planet's gravitational tug shows up as a slight shift in the star's spectrum.

Another approach aims to detect the slight dimming of starlight that occurs when a planet eclipses its star. This latest method takes advantage of gravity's ability to bend and magnify light, much like a magnifying glass. The solar system the team discovered passed in front of a more-distant star. Its brightness briefly spiked as the planet-bearing star passed in front of it. Then it spiked again, though less intensely, as the planet passed in front. These unique spikes allowed astronomers to estimate the planet's orbit and mass.

The team, led by Jean-Phillipe Beaulieu of the Institut d'Astrophysique in Paris, unveiled its results in Thursday's issue of the journal Nature.

Another team, meanwhile, has found bright disks of icy, rocky debris around two stars much closer to home - only about 60 light-years from Earth. Detected with the Hubble Space Telescope, the disks appear to be similar to the Kuiper Belt, a broad band of construction rubble left over from the formation of the solar system some 4.6 billion years ago. The Kuiper Belt lies just beyond Neptune and stretches to a distance of from 30 to 50 astronomical units (a.u.) from the sun. (One a.u. is 93 million miles.)

Debris disks have been observed at visible wavelengths on only seven stars, though disks' infrared signatures have been detected around many more. Yet these recently discovered disks appear to be old enough to have become set in their orbits. The research appears in the current issue of Astrophysical Journal Letters.

Efforts to unravel the mysteries of how solar systems form - and whether other solar systems harbor life - are experiencing a burst in activity as long-running ground-based surveys bear fruit and new planet-hunting techniques are introduced. Researchers at the University of Florida, for example, say they've developed a new detector that should allow them to examine several hundred thousand stars over the next 20 years, compared with 3,000 stars that scientists have scanned over the past 20 years.

"We're looking at young stars to see how planets are forming, at their dust production, and we're looking at older systems and seeing planets in their final, bizarre orbits," says Benjamin Bromley, a planetary scientist at the University of Utah. The only way to answer the "life elsewhere" question is by "piecing together how this process of planet formation happens. It may seem unexciting to pick up just one more planet, but with each additional planet, our understanding of the whole process improves."